Former Trump political strategist Steve Bannon has been subpoenaed by special counsel Robert Mueller in a sign that the investigation is moving closer to the president.

The New York Times reports that Bannon was issued a subpoena last week and it is “the first time Mr. Mueller is known to have used a grand jury subpoena to seek information from a member of Mr. Trump’s inner circle.”

Other Trump associates have testified either before the grand jury or directly to investigators without needing to be subpoenaed, and the Times writes that “it was not clear why Mr. Mueller treated Mr. Bannon differently than the dozen administration officials who were interviewed in the final months of last year and were never served with a subpoena.”

Bannon is testifying before the House Intelligence Committee on Tuesday, and there has been speculation that he might turn on the president. In Michael Wolff’s book “Fire and Fury: Inside the Trump White House,” Bannon said he thought Mueller would nail Jared Kushner and Donald Trump Jr. on money laundering charges — and then use them to get to President Trump.

In the wake of those comments, Trump publicly raged at Bannon and the Trump White House urged Breitbart to consider firing Trump’s one-time political ally.